If you configure gvfs with --enable-libmtp but --disable-udev, the configure script disables MTP support (it confirms this in the textual output at the end of configure, but it doesn’t actually fail the configure process with a nonzero exit code). So, USE=mtp should require USE=udev, so this is documented and obvious to the user, rather than silently installing gvfs with USE=mtp but not getting MTP support.
+*gvfs-1.20.3 (05 Sep 2014)
+ 05 Sep 2014; Pacho Ramos <firstname.lastname@example.org> +gvfs-1.20.3.ebuild,
+ -files/gvfs-1.12.3-libcdio-0.84.patch, -files/gvfs-1.16.3-emit-signal.patch,
+ -files/gvfs-1.2.2-gphoto2-stricter-checks.patch, -gvfs-1.18.3.ebuild,
+ -gvfs-1.20.1.ebuild, metadata.xml:
+ Version bump, use upstream rules to install bach-completion file, 'mtp' needs
+ 'udev' support (#521472 by Christopher Head), drop old.